This invention relates to a novel process for the preparation of dehydroacetic acid, a compound useful as a fungicide, bactericide, and plasticizer as well as an intermediate in organic syntheses.
Dehydroacetic acid has been prepared by a variety of methods, many of which are based on the dimerization of diketene, generally at elevated temperatures and in the presence of a catalyst [See U.S. Pat. Nos. 2,229,204, 2,849,456 and 3,483,224]. Additionally, F. Arndt, Organic Syntheses, Coll. Vol. III, 1955, 231-233, reports the preparation of dehydroacetic acid by the condensation of ethyl acetoacetate in the presence of sodium bicarbonate at a temperature of 200.degree. to 210.degree. C. Disclosed by Carroll and Bader in the "Journal of American Chemical Society," Vol. 75, 5400-02, 1953, is the preparation of dehydroacetic acid in 51% yield by heating 2,2,6-trimethyl-4H-1,3-dioxin-4-one in toluene in the presence of calcium acetate for 5 hours.
According to the present invention, dehydroacetic acid is obtained in excellent yield, in short reaction time and with a minimum of reagents. The process of my invention begins not with the diketene as in many existing methods but with the compound 2,2,6-trimethyl-4H-1,3-dioxin-4-one, having the formula ##STR1## Surprisingly, I have found that dehydroacetic acid can be produced in nearly quantitative yields (up to about 99% yield) simply by heating compound I.